Nowadays electronic devices have almost become necessary goods in people's life. When the electronic device is in use, heat energy is generated. Hence the electronic device usually has radiator fans. The radiator fan has rotary vanes to discharge heated air and maintain the electronic device at a desired temperature for normal operation. For more delicate electronic devices, a great amount of heat energy is generated. Hence increasing heat dissipation efficiency of the electronic device is important.
Referring to FIG. 1 for a chart showing the relationship between pressure and airflow. During tests, the radiator fan is located in a housing tank. The housing tank has an opening. When the opening is completely blocked, a maximum pressure difference (minimum airflow) at two ends of the radiator fan is obtained. On the contrary, when the opening is fully open, a minimum pressure difference (maximum airflow) at two ends of the radiator fan is obtained. The curves in FIG. 1 indicate the maximum pressure overcome by the radiator fan. The cross points of the curves and the pressure axis are the maximum pressure differences, while the cross points of the curves and the airflow axis are the minimum pressure differences. The areas covered by the curves and two axes are the discharged airflow of the radiator fan. For an electronic device which has two radiator fans installed in series, a best heat dissipation efficiency may be obtained as shown by curve A. When the electronic device has only one radiator fan, it has a second best heat dissipation efficiency as shown by curve B. When the electronic device has two radiator fans installed in series, and the radiator fan close to the interior of the electronic device stops operation, it has a second worst heat dissipation efficiency as shown by curve C. When the electronic device has two radiator fans installed in series, and the radiator fan close to the exterior of the electronic device stops operation, it has the worst heat dissipation efficiency as shown by curve D.
In most circumstances, electronic devices have two radiator fans installed in series to improve heat dissipation efficiency. However, if any one of the radiator fans stops operation, the resulting heat dissipation efficiency is lower than only one radiator fan is installed. This is because air inside the electronic device cannot flow outwards in the positive direction. The pressure difference between the interior and exterior increases, and heat dissipation efficiency drops. To remedy this drawback, there is a design to increase the rotation speed of one radiator fan when another radiator fan has stopped operation so that the heat dissipation efficiency of the radiator with two series radiator fans (curve A) is close to the heat dissipation efficiency of the radiator with one radiator fan stopping operation (curve C or D). Namely, the difference of heat dissipation efficiency is not excessively large when the two radiator fans are operating and when any of the radiator fans is not operable. Of course, to make the curve A close to the curves C and D also is the direction the industry is pursuing.